FIG. 1 schematically illustrates a magnetically deflected cathode ray tube (CRT) of a type which is known in the art. The CRT generally comprises a funnel-shaped portion 21 which terminates in phosphor screen 25, and a cylindrical neck portion 11 housing electron gun apparatus 10. Electron gun apparatus 10 generates and focuses an electron beam to produce a luminous image on the phosphor screen. Cathode 12 emits a stream of electrons forming electron beam 15, which successively passes through apertures in first grid 14 and second grid 16. Thereafter, the electron beam passes through an aperture at one end of anode cylinder 18, and it typically traverses a series of beam focusing and/or correction members 20. For example, a series of wafer electrodes may be provided to correct astigmatism produced by nonuniformities in the electromagnetic deflection field, as taught in U.S. Pat. No. 4,672,276. Electron beam 15 subsequently traverses focus electrode 22 and magnetic deflection yoke 24 and finally impinges o and illuminates the phosphor screen. An important objective in many CRT applications is to generate an electron beam providing a high intensity, sharply focused illuminated spot over the entire CRT screen. To achieve this objective, the electron beam must traverse the electron gun and funneling portion of the CRT without deviating from the desired beam path.
The present invention is directed to the electron gun portion of the CRT apparatus. First and second grids 14 and 16, respectively, have voltages applied thereto. Changes in the voltages applied to the cathode and the first grid with respect to one another vary the magnitude of electron emissions, and thereby vary the intensity of illumination on the phosphor screen. The voltage applied to second grid 16 determines the cathode voltage at which electrons are emitted. Precise repeatable spacing between the cathode, the first grid and the second grid, and precise alignment of apertures in the first and second grids are critical factors. Small misalignments of the first and second grids result in significant distortion of the electron beam from its desired beam path axis. For example, a misalignment of the first and second grid apertures of 0.5 mil (0.0005 inch) produces a 30% axial misalignment of the beam from its desired beam path axis at the limiting aperture and focus assembly of a high resolution electron gun. Beam misalignment and distortion of this type typically results in "banding", which is manifested by the formation of bright lines at intervals in the rastered portion of the CRT screen. Banding reduces the clarity and resolution of a CRT display.
FIG. 2 illustrates a conventional prior art first grid (G1)/second grid (G2) assembly. Cathode assembly 30 comprises cathode sleeve 31 having cathode cap 32 mounted at one end, and cathode 33 at the terminal end thereof providing emission of the electron beam. Cathode assembly 30 is rigidly mounted in non-conductive cathode support member 34 and provided with the necessary support means and electrical connections, as is well known in the art. First grid (G1) assembly 35 comprises a generally flat grid layer 36 with central aperture 37 and grid cap 38 extending generally at a right angle from the periphery of flat grid layer 36. A spacer 39 is preferably provided to locate the cathode assembly with respect to the first grid assembly in the axial and radial directions.
Second grid (G2) 40 is generally flat and has central aperture 41 provided therein and aligned with aperture 37. Second grid 40 is arranged parallel to and spaced a distance from first grid (G1). The anode assembly, which has a surface arranged parallel to and spaced apart from second grid (G2) and a central aperture aligned with apertures 37 and 41 is not shown. As electron beam 15 is emitted from cathode 33 and traverses the multiple grid and anode apertures during operation of the electron gun and CRT, it is important that the apertures in the grid and anode assemblies are properly aligned to assure that the electron beam is not deflected from its predetermined, calculated path.
In conventional, prior art electrode gun assemblies, first and second grids (G1 and G2) are fixed in relationship to one another by means of support pins 42 mounted in glass rods 44, as shown in FIG. 2. A plurality of glass rods 44 (generally four) having support pins bonded therein are generally arranged radially around the periphery of the G1 cap. This arrangement serves to space the grids from one another at the desired axial spacing and to align the grids radially with respect to one another. Glass rods 44 are typically heated, and support pins 42 mounted on both grids are embedded therein while the glass is softened by heating. The G1/G2 assembly is cooled, and central apertures 37 and 41 are subsequently provided in both grids G1 and G2, typically by EDM (electron discharge machine), so that the grid apertures are precisely aligned. The cathode assembly is thereafter mounted in the first grid (G1) assembly. The anode assembly is similarly mounted on a plurality of radially arranged glass rod supports which typically extend substantially the length of the electron gun assembly and provide structural support for many of the electron gun components.
Misalignment of the first (G1) and second (G2) grid apertures 37 and 41, respectively, generally occurs because the grid (G1/G2) assembly shown in FIG. 2 lacks mechanical strength and rigidity. During mounting of support pins 42 in glass rods 44, the glass rods are softened by flame heating to permit embedment. As the assembly is cooled, the metallic support pins and glass rods have different coefficients of thermal expansion, and the metallic support pins contract more than the surrounding glass surface. The support pins become trapped rather than bonded in the glass rods. The grid (G1/G2) assembly therefore does not exhibit good mechanical strength, and movement of the first and second grids relative to one another may occur during and/or after provision of the grid apertures. Movement of the first and second grids relative to one another frequently occurs during mounting of the cathode assembly, which creates substantial misalignments of the grid apertures.
Accordingly, it is an objective of the present invention to provide a grid (G1/G2) assembly which is characterized by mechanical strength and rigidity.
It is another objective of the present invention to provide a grid (G1/G2) assembly in which the grid apertures remain accurately and precisely aligned during assembly and operation of the electron gun and CRT.
It is yet another objective of the present invention to provide methods for reducing deviation and distortion of the electron beam in the electron gun assembly of the CRT.